The present invention relates generally to the field of non-invasive imaging and more specifically to the field of computed tomography. In particular, the present invention relates to improving z-axis coverage using collimation and timing techniques.
CT scanners operate by projecting fan shaped or cone shaped X-ray beams from an X-ray source. The X-ray source emits X-rays at numerous angles relative to an object being imaged, such as a patient, which attenuates the X-ray beams as they pass through. The attenuated beams are detected by a set of detector elements, which produce signals representing the attenuation of the incident X-ray beams. The signals are processed to produce data representing the line integrals of the attenuation coefficients of the object along the X-ray paths. These signals are typically called “projection data” or just “projections”. By using reconstruction techniques, such as filtered backprojection, useful images may be formulated from the projections. The images may in turn be associated to form a volume rendering of a region of interest. In a medical context, pathologies or other structures of interest may then be located or identified from the reconstructed images or rendered volume.
It is generally desirable to develop CT scanners with high spatial and temporal resolution, good image quality, and good coverage along the z-axis, i.e., the long or rotational axis of the CT scanner. To meet some or all of these objectives, it may be desirable to increase the coverage provided by the detector, thereby allowing greater scan coverage in one or more dimensions. For example, z-axis coverage of the detector may be improved by increasing the number of rows of detector elements in the detector.
This approach has lead to the development of CT systems with larger detectors. Larger detectors, however, may be undesirable for a variety of reasons. For example, as one might expect, larger detectors are both more costly and more difficult to produce. In addition, the mechanical subsystem responsible for supporting and/or rotating a larger detector may also need to be larger and more complex and/or may be subject to greater mechanical stress. Furthermore, larger detectors are associated with larger cone angles, i.e., the angle between the source and the detector periphery. The increased cone angle between the source and detector periphery is associated with increased cone beam artifacts in the reconstructed images, particularly in axial scan modes. When the cone angle increases beyond a certain limit, the degradation of the image quality may become severe. For this reason, it may be difficult to increase the z-axis coverage by simply increasing the coverage of the detector in the direction of the z-axis.
One alternative has been to move the source further from the object or patient, to minimize magnification, thereby reducing cone angle. Increasing the source distance, however, may not be physically feasible. Another alternative has been to increase the detector element size along the periphery to reduce or maintain the number of detector channels, i.e., the total number of detector elements. Increasing the size of detector elements, however, may reduce the overall resolution of the CT system beyond what is desired or acceptable. A further alternative has been to employ multiple X-say sources along the z-axis that are sequentially activated. In this configuration, however, each source illuminates the full detector when active, resulting in large cone angles and the generation of cone beam artifacts in the reconstructed images. A technique for improving z-axis coverage without image degradation may therefore be desirable.